Running two radiator caps?| Grassroots Motorsports forum |

CarRacer Reader
2/15/15 5:47 p.m.

I had to change the thermostat housing to clear the firewall on my engine swap and in the process add another radiator cap to the system. Is this a problem? I'm planning on only running a catch can from the rear cap and plugging the outlet on the front cap, is this a stupid idea?

Thanks.

Don49 HalfDork
2/15/15 5:51 p.m.

Shouldn't be a problem.

Boost_Crazy Reader
2/15/15 7:25 p.m.

Make sure the cap at the highest point is the cap that feeds the overflow. Use a non relief cap on the other one.

kb58 Dork
2/15/15 7:58 p.m.

You'll be fine - just do as Boost Crazy recommended.

HappyAndy UltraDork
2/15/15 8:08 p.m.

Boost_Crazy wrote: Make sure the cap at the highest point is the cap that feeds the overflow. Use a non relief cap on the other one.

Or, if for some reason a non relief cap is not available, the one at the lower point should have the higher relief pressure of the two, preferably by several psi.

wheels777 Dork
2/15/15 8:24 p.m.

I usually run two caps. Both get routed vent lines to the overflow. The cap at the higher elevation get a lower pressure rated cap.

mr2peak HalfDork
2/15/15 8:25 p.m.

How about some details. Turbo 4 e30s are cool

carbon Dork
2/15/15 9:40 p.m.

Should be easy to purge air.

amg_rx7 SuperDork
2/16/15 12:33 a.m.

HappyAndy wrote:
Boost_Crazy wrote: Make sure the cap at the highest point is the cap that feeds the overflow. Use a non relief cap on the other one.
Or, if for some reason a non relief cap is not available, the one at the lower point should have the higher relief pressure of the two, preferably by several psi.

That's how Mazda did it on the FD. They also added an Air Separation Tank like the more modern Vette's have.